1. Technical Field of the Invention
This invention relates generally to motorcycles, and more specifically to final drive chain solutions for them.
2. Background Art
FIG. 1 illustrates a conventional motorcycle 10 including a frame 12 holding an engine 14 which includes a gearbox 16. The final output of the gearbox is a shaft 18 to which is coupled a front sprocket 20. The front sprocket drives a chain 22, which in turn drives a rear sprocket 24 which is coupled to a rear wheel 26. The rear wheel is pivotably coupled to the frame or to the engine by a swingarm 28. A shock/spring unit 30 supports the frame or engine on the swingarm.
Under acceleration, the engine is supplying tremendous torque through the front sprocket, and the top segment 22T of the chain is under an extraordinary amount of strain, while the bottom segment 22B of the chain is relatively slack. Because the chain is located to one side of the rear wheel, and thus closer to one side of the swingarm (the right side, as illustrated) than to the other side (Where the brake rotor is located), the pulling force of the top segment of the chain applies a very asymmetric load on the swingarm, tending to pull it rather strongly toward the chain side.
In order to avoid undesirable twisting and lateral flexing of the swingarm, the swingarm is made stronger, taller, thicker, and heavier. This increased mass reduces the acceleration of the motorcycle, reduces the ability of the rear wheel to track over bumps, and decreases the performance of the rear shock. Additionally, because the suspension components of the motorcycle are oriented in a substantially planar orientation with regard to the vertical, longitudinal plane of the motorcycle (i.e. the plane of the paper in FIG. 1), and because motorcycles lean significantly over on their sides when cornering, during cornering the suspension components are not in an orientation conducive to tracking vertical bumps and irregularities in the road surface. A significant fraction of the motorcycle rear end's vertical tracking in corners should, ideally, be provided by flex in the swingarm itself. Unfortunately, this is directly at odds with the need to stiffen the swingarm enough to avoid chain-induced flex and twist.
What is needed, then, is an improved motorcycle which has a rear end in which the swingarm can be more readily utilized and tuned as a meaningful lateral suspension component.